Nested table in fixed-width column gets squashed

[rudolfwalter]

Version/Branch of Dear ImGui:

Version: 1.89.7 Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_sdl2.cpp + imgui_impl_sdlrenderer2.cpp Compiler: MSVC Operating System: Windows 10

My Issue/Question:

I'm trying to define a somewhat complicated layout using nested tables, and I'm running into a problem.

I want to put a nested table into a fixed-width column of a parent table. My expectation was that the fixed-width column would neatly hug the nested table, just like it does with simple widgets. What happens instead is that the nested table gets squashed to (near-)zero width, at least visually.

Sizing-wise, the nested table still provides a minimum preferred width to the window – ie, in the below example, you can see the length of the label of the checkbox affecting the window if you add the AlwaysAutoResize flag.

Note: I tried this on docking branch versions 1.89.1 and 1.89.7, it happens on both.

Screenshots/Video

Result of the example code below:

Standalone, minimal, complete and verifiable example:

ImGui::Begin("Bug");
if (ImGui::BeginTable("outer", 2))
{
    ImGui::TableSetupColumn("1", ImGuiTableColumnFlags_WidthFixed);
    ImGui::TableSetupColumn("2");

    ImGui::TableNextColumn();
    if (ImGui::BeginTable("inner", 1))
    {
        ImGui::TableNextColumn();
        bool b = true;
        ImGui::Checkbox("mycheckbox", &b); //BUG: this gets squashed

        ImGui::EndTable();
    }

    ImGui::TableNextColumn();
    ImGui::Text("hello");

    ImGui::EndTable();
}
ImGui::End();

[ocornut]

Hello,

I don't have resources to go in depth into this now, but at lowest-level this is somewhat similar to the cause leading to #6421. Except that here "interestingly" it leads to a secondary issue as not only you are not getting the layout you want, but the checkbox doesn't look clipped at the emitted width bubbled up in a way leading to draw call merge across both tables.

I'm sure there is a solution but it's a rather tricky thing to untangle.

( Random notes for my own records:

• in EndTable(), nested table clamps its reported CursorMaxPos.x with table->OuterRect.Max.x.
• Analogous to Y axis might need to support a scheme where OuterRect.Max.x stays FLT_MAX until end of table but it opens up a bunch of things.
• As a result, while regular tables rely on clipping rectangle to match that maximum extent, here the container/parent table uses this value emitted its nested table to run its own draw call merge and cancel out the clipping rectangle from nested table.

In EndTable() if you replace:

outer_window->DC.CursorMaxPos.x = ImMax(backup_outer_max_pos.x, ImMin(table->OuterRect.Max.x, table->OuterRect.Min.x + table->ColumnsAutoFitWidth));

With

outer_window->DC.CursorMaxPos.x = ImMax(backup_outer_max_pos.x, table->OuterRect.Min.x + table->ColumnsAutoFitWidth);

It will appear to fix your specific issue but has other side-effects.

I will investigate this eventually, the failing table_reported_size_outer test has a test bed (right-click "Run GUI Func") showcase the problem:

Here with a "small" window width (the "container"):

• window can fit about 61 worth of contents
• if we pass -FLT_MIN width BeginTable() aka right-align, the table is 61 large.
• if we pass 0.0f width to BeginTable() it default to the same (right-align) which perhaps is the crux of the problem.
• the item inside table is 100 large.
• it is correct we don't see a Scrollbar inside the table (because no ScrollX flag) and it is correct we don't see a Scrollbar inside the parent window if the table is 61 large.

I suspect the solution is:

• treat outer_size.x == 0 differently from outer_size.x == -FLT_MIN.
• which may involve clarifying/improving/reworking ImGuiTableFlags_NoHostExtendX (which at first glance is confusingly documented).

)

[rudolfwalter]

Thank you for all those details!

The change to EndTable() that you suggested does indeed fix this particular issue; however I'm not familiar enough with the internals of dear imgui to be able to imagine what side effects it might have.

The work-around I came up with on my own was to replace the EndTable() of the nested table with the following:

float table_width = ImGui::GetCurrentContext()->CurrentTable->ColumnsAutoFitWidth;
ImGui::PushStyleVar(ImGuiStyleVar_ItemSpacing, ImVec2(0, 0));
ImGui::EndTable();
ImGui::Dummy(ImVec2(table_width, 0));
ImGui::PopStyleVar();

This has the potential advantage of being localized, instead of affecting all tables. (ColumnsAutoFitWidth is private of course, but to me that seems neither better nor worse than patching the source code.)